The present invention relates to the copolymerization of styrene and acrylonitrile by the emulsion process.
Styrene acrylonitrile copolymers are used currently in a wide variety of products including telephone casings and drain piping.
Styrene acrylonitrile (SAN) copolymers are currently produced by three processes, the emulsion process, the suspension process and the continuous mass process. In the emulsion process, with which this application is concerned, the conversion of the monomers to the copolymers goes to a completion of only 97% of theoretical. The unreacted 3% is effectively wasted in economic terms, and discharge of the unreacted monomers constitutes a problem with respect to pollution, especially in terms of unreacted acrylonitrile which is highly toxic. No fundamental reason has been found why this copolymerization reaction cannot go to completion.
The problem of providing an acrylonitrile copolymer free of residual monomer has been dealt with previously. Thus, in U.S. Pat. Nos. 4,197,400 and 4,241,203, heating of the copolymer is recommended in order to reduce the residual monomer content to a very low level.